In the study, Decety and doctoral student Thierry Chaminade used positron emission tomography (PET) scans to explore what brain systems were activated while people watched videos of actors telling stories that were either sad or neutral in tone. The neutral stories were based on everyday activities such as cooking and shopping. The sad stories described events that could have happened to anyone, such as a drowning accident or the illness of a close relative. The actors were videotaped telling the stories, which lasted one to two minutes, with three different expressions – neutral, happy or sad.

Decety and Chaminade found that, as people watched the videos, different brain regions were activated depending on whether an actor's expressions matched the emotional content of the story.

When the story content and expression were congruent, neural activity increased in emotional processing areas of the brain – the amygdala and the adjacent orbitofrontal cortex and the insula. In addition, increased activation also was noted in what neuroscientists call the "shared representational" network which includes the right inferior parietal cortex and premotor cortex. This network refers to brain areas that are activated when a person has a mental image of performing an action, actually performs that action or observes someone else performing it.

However, these emotional processing areas were suppressed when the story content and expression were mismatched, such as by having a person smile while telling about his mother's death. Instead, activation was centered in the ventromedial prefrontal cortex and superior frontal gyrus, regions that deal with social conflict.

After watching each video clip, the 12 subjects in the study also were asked to rate the storyteller's mood and likability. Not surprisingly the subjects found the storytellers more likable and felt more sympathetic toward them when their emotional expression matched a story's content than when it did not.

"Sympathy is a very basic way in which we are connected to other people," said Decety. "We feel more sympathy if the person we are interacting with is more like us. When people act in strange ways, you feel that person is not like you.

"It is important to note that the emotional processing network of the brain was not activated when the subjects in our study watched what we would consider to be inappropriate social behavior. Knowing how the brain typically functions in people when they are sympathetic will lead to a better understanding of why some individuals lack sympathy."

Imagine a future where people can be genetically engineered to lack sympathy. I think the technical ability to eventually do this is a matter of when, not if. PET Scans might be used to detect the equivalent of Blade Runner replicants.

If SD-6 started using this technique to look for sympathy then Sydney Bristow of Alias could be in a whole world of trouble. Still, it would be hard to word the questions to trip her up since she is supposed to believe that by working for SD-6 she's already working for the CIA.